1. Field of the Invention
The present invention relates generally to a clipper circuit for a power transistor for absorbing surge voltage. More specifically, the invention relates to a clipper circuit which is suitable for simplification of a power transistor circuit and whereby allowing the power transistor circuit to be made satisfactorily compact.
2. Description of the Background Art
Power transistor circuits have been used in various electric source units as large current control elements. For example, a power transistor may be employed as a switching element in an inverter circuit. In such cases, it is a known technique to provide a clipper circuit in the power transistor circuit for absorbing surge voltages which are generated upon shutting off of the current, thereby protecting the power transistor.
The conventional power transistor circuit, which is employed in a single phase inverter circuit, is associated with a power source clipper circuit interposed between the power transistor circuit and a power source. The power source clipper circuit is designed to absorb surge voltage generated in the power source. The power transistor circuit also incorporates a clipper circuit. The clipper circuit in the power transistor circuit cooperates with the power source clipper circuit at the time of current shut off so that surges having high dv/dt are absorbed by the clipper circuit in the power transistor circuit and large energy surges are absorbed by the power source clipper circuit.
In general, the clipper circuit is incorporated in the power transistor circuit, a series circuit comprised of a capacitor and a diode, and a discharge resistor. The capacitor-diode circuit is connected in parallel to a collector-emitter circuit of a power transistor. When the power transistor is turned OFF, collector current flows through the capacitor-diode series circuit. In the transition, the current is divided by the capacitor and the diode. By dividing the current, voltage surges due to inductance of the wiring and so forth can be absorbed to lower the dv/dt of voltage variations in collector-emitter circuit of the power transistor. However, in the vicinity of the zero-crossing of the divided current, high frequency oscillation tends to be caused. This high frequency oscillation causes turning ON of the power transistor which, in turn causes shorting breakage upon turn-on of the other power transistor circuit. In order to suppress the voltage oscillation, a snubber circuit is provided in the power transistor circuit.